1. Field of the Invention
The invention relates to a continuous process for the preparation of d,l-menthol in the presence of a fixed bed catalyst which comprises palladium, ruthenium or rhodium or a mixture of these elements as active constituents, and furthermore comprises promoters, and the support of which is doped with a rare earth metal and manganese.
The d,l-menthol prepared can be employed either in the racemic form or, after resolution of the racemate, in the form of its l - or d-antipode in the perfumes, flavourings and medicaments industry.
2. Description of the Related Art
It is known to prepare d,l-menthol by discontinuous catalytic hydrogenation of thymol and other compounds which have the carbon skeleton of menthol having at least one double bond in the 4-position and are substituted by oxygen. The preparation of d,l-menthol by racemisation and/or rearrangement of d-menthol and the optically active or inactive stereoisomers of menthol is likewise known and can be carried out, for example, by heating these compounds with hydrogen in the presence of a hydrogenation catalyst. A crude product which comprises d,l-menthol, d,l-neomenthol, d,l-isomenthol and d,l-neoisomenthol in a ratio of about 6:3:1:0.2 parts by weight, corresponding to equilibrium being established, is in general obtained here in all cases, regardless of the starting material both in the catalytic hydrogenation and in the racemisation, as a result of the epimerisation which takes place during such processes. In addition to the abovementioned stereoisomers, however, by-products such as menthone and isomenthone, and furthermore methenone and hydrocarbons, such as menthenes and menthanes are additionally formed. The formation of these by-products, which can be reduced and/or rearranged to give d,l-menthol only with difficulty, if at all, causes the considerable disadvantages of processes of the prior art of the type described; for example, about 5% of hydrocarbons which cannot be reused are obtained in the process described in U.S. Pat. No. 2,843,636.
The disadvantages described are of particular significance if attempts are made to carry out these processes, which are described only for the discontinuous procedure, continuously by separating off the d,l-menthol from the product stream by means of physical separation processes and circulating the by-products. In this procedure, these undesirable by-products become concentrated in the circulating stream very quickly such that a continuous procedure based on this process is uneconomical. It is known from German Auslegeschrift 23 14 813 that d,l-menthol can be prepared by catalytic hydrogenation of compounds which have the carbon skeleton of menthol having at least one double bond and are substituted by oxygen in the 3-position relative to the methyl group, and/or by rearrangement of optically active or inactive stereoisomers of menthol at elevated temperature and under pressure if the hydrogenation is carried out continuously using a cobalt catalyst, arranged in a fixed bed, having a manganese content of 10 to 40% by weight, based on the total amount of cobalt and manganese, the amount of hydrogen per mol of starting material being at least 10 times the amount required for hydrogenation of a benzene nucleus; this reaction is carried out in the temperature range from 170.degree. to 220.degree. C. and under a pressure of at least 25 bar. A cobalt catalyst having a content of 15 to 30, in particular 20 to 25% by weight of manganese is preferably used.
However, the use of the cobalt/manganese catalyst according to German Auslegeschrift 23 14 813 mentioned is industrially expensive and pollutes the environment. Moreover, the German Auslegeschrift mentioned contains no information on the mechanical stability and life of this catalyst, and this information is of decisive importance for industrial production of d,l-menthol.